Method and system for balancing reference signal powers across OFDM symbols

ABSTRACT

A base station includes a reference signal allocator that allocates a first layer of dedicated reference signals and a second layer of reference signals to the same resource elements in a first resource block. The reference signals are allocated to two adjacent resource elements corresponding to a first OFDM symbol and a second OFDM symbol on a first, second, and third subcarriers of the first resource block. The base station also includes a reference signal multiplexer that multiplexes the first layer with the second layer. A first cover code W 1  is applied to the first layer. A second cover code W 2 , different from the first cover code, is applied to the second layer in a first and third subcarriers, and a variation of the second cover code W 2 ′ is applied to the second layer in a second subcarrier.

CROSS-REFERENCE TO RELATED APPLICATION(S) AND CLAIM OF PRIORITY

The present application is a continuation of U.S. Non-Provisional patentapplication Ser. No. 14/312,346 filed Jun. 23, 2014, which is acontinuation of U.S. Non-Provisional patent application Ser. No.12/941,848 filed Nov. 8, 2010, now U.S. Pat. No. 8,761,087, which claimspriority to U.S. Provisional Patent Application No. 61/260,307, filedNov. 11, 2009, entitled “MULTI-LAYER BEAMFORMING METHODS IN WIRELESSCOMMUNICATION SYSTEMS” and to U.S. Provisional Patent Application No.61/323,240, filed Apr. 12, 2010, entitled “MULTI-LAYER BEAMFORMINGMETHODS IN WIRELESS COMMUNICATION SYSTEMS.” The above-identified patentdocuments are hereby incorporated by reference.

TECHNICAL FIELD

The present application relates generally to wireless communicationsand, more specifically, to a method and system for reference signal (RS)pattern design.

BACKGROUND

In 3^(rd) Generation Partnership Project Long Term Evolution (3GPP LTE),Orthogonal Frequency Division Multiplexing (OFDM) is adopted as adownlink (DL) transmission scheme.

SUMMARY

A base station is provided. The base station includes a downlinktransmit path comprising circuitry configured to transmit a plurality ofreference signals in a first resource block. The resource blockcomprises S OFDM symbols, each of the S OFDM symbols comprises Nsubcarriers, and each subcarrier of each OFDM symbol comprises aresource element. The base station also includes reference signalallocator configured to allocate a first layer of the reference signalsand a second layer of the reference signals to the same resourceelements in the first resource block. The reference signals are (i)allocated to a first group of two adjacent resource elementscorresponding to a first OFDM symbol and a second OFDM symbol on a firstsubcarrier of the first resource block, (ii) allocated to a second groupof two adjacent resource elements corresponding to the first OFDM symboland the second OFDM symbol on a second subcarrier of the first resourceblock, and (iii) allocated to a third group of two adjacent resourceelements corresponding to the first OFDM symbol and the second OFDMsymbol on a third subcarrier of the first resource block. The basestation further includes a reference signal multiplexer configured tomultiplex the first layer of the reference signals with the second layerof the reference signals by applying a first cover code W1 to the firstlayer of the reference signals, applying a second cover code W2,different from the first cover code, to the second layer of thereference signals on the first subcarrier and the third subcarrier, andapplying a variation of the second cover code W2′ to the second layer ofthe reference signals on the second subcarrier.

A method of operating a base station is provided. The method includestransmitting a plurality of reference signals in a first resource block.The resource block comprises S OFDM symbols, each of the S OFDM symbolscomprises N subcarriers, and each subcarrier of each OFDM symbolcomprises a resource element. The method also includes allocating afirst layer of the reference signals and a second layer of the referencesignals to the same resource elements in the first resource block. Thereference signals are (i) allocated to a first group of two adjacentresource elements corresponding to a first OFDM symbol and a second OFDMsymbol on a first subcarrier of the first resource block, (ii) allocatedto a second group of two adjacent resource elements corresponding to thefirst OFDM symbol and the second OFDM symbol on a second subcarrier ofthe first resource block, and (iii) allocated to a third group of twoadjacent resource elements corresponding to the first OFDM symbol andthe second OFDM symbol on a third subcarrier of the first resourceblock. The method further includes multiplexing the first layer of thereference signals with the second layer of the reference signals byapplying a first cover code W1 to the first layer of the referencesignals, applying a second cover code W2, different from the first covercode, to the second layer of the reference signals on the firstsubcarrier and the third subcarrier, and applying a variation of thesecond cover code W2′ to the second layer of the reference signals onthe second subcarrier.

A subscriber station is provided. The subscriber station includes adownlink receive path that includes circuitry configured to receive aplurality of reference signals in a first resource block. The resourceblock comprising S OFDM symbols, each of the S OFDM symbols comprises Nsubcarriers, and each subcarrier of each OFDM symbol comprises aresource element. A first layer of the reference signals and a secondlayer of the reference signals are allocated to the same resourceelements in the first resource block. The reference signals are (i)allocated to a first group of two adjacent resource elementscorresponding to a first OFDM symbol and a second OFDM symbol on a firstsubcarrier of the first resource block, (ii) allocated to a second groupof two adjacent resource elements corresponding to the first OFDM symboland the second OFDM symbol on a second subcarrier of the first resourceblock, and (iii) allocated to a third group of two adjacent resourceelements corresponding to the first OFDM symbol and the second OFDMsymbol on a third subcarrier of the first resource block. The firstlayer of the reference signals is multiplexed with the second layer ofthe reference signals. A first cover code W1 is applied to the firstlayer of the reference signals, a second cover code W2, different fromthe first cover code, is applied to the second layer of the referencesignals on the first subcarrier and the third subcarrier, and avariation of the second cover code W2′ is applied to the second layer ofthe reference signals on the second subcarrier.

A method of operating a subscriber station is provided. The methodincludes receiving a plurality of reference signals in a first resourceblock. The resource block comprises S OFDM symbols, each of the S OFDMsymbols comprises N subcarriers, and each subcarrier of each OFDM symbolcomprises a resource element. A first layer of the reference signals anda second layer of the reference signals are allocated to the sameresource elements in the first resource block. The reference signals are(i) allocated to a first group of two adjacent resource elementscorresponding to a first OFDM symbol and a second OFDM symbol on a firstsubcarrier of the first resource block, (ii) allocated to a second groupof two adjacent resource elements corresponding to the first OFDM symboland the second OFDM symbol on a second subcarrier of the first resourceblock, and (iii) allocated to a third group of two adjacent resourceelements corresponding to the first OFDM symbol and the second OFDMsymbol on a third subcarrier of the first resource block. The firstlayer of the reference signals is multiplexed with the second layer ofthe reference signals. A first cover code W1 is applied to the firstlayer of the reference signals, a second cover code W2, different fromthe first cover code, is applied to the second layer of the referencesignals on the first subcarrier and the third subcarrier, and avariation of the second cover code W2′ is applied to the second layer ofthe reference signals on the second subcarrier.

Before undertaking the DETAILED DESCRIPTION below, it may beadvantageous to set forth definitions of certain words and phrases usedthroughout this patent document: the terms “include” and “comprise,” aswell as derivatives thereof, mean inclusion without limitation; the term“or,” is inclusive, meaning and/or; the phrases “associated with” and“associated therewith,” as well as derivatives thereof, may mean toinclude, be included within, interconnect with, contain, be containedwithin, connect to or with, couple to or with, be communicable with,cooperate with, interleave, juxtapose, be proximate to, be bound to orwith, have, have a property of, or the like; and the term “controller”means any device, system or part thereof that controls at least oneoperation, such a device may be implemented in hardware, firmware orsoftware, or some combination of at least two of the same. It should benoted that the functionality associated with any particular controllermay be centralized or distributed, whether locally or remotely.Definitions for certain words and phrases are provided throughout thispatent document, those of ordinary skill in the art should understandthat in many, if not most instances, such definitions apply to prior, aswell as future uses of such defined words and phrases.

BRIEF DESCRIPTION OF THE DRAWINGS

For a more complete understanding of the present disclosure and itsadvantages, reference is now made to the following description taken inconjunction with the accompanying drawings, in which like referencenumerals represent like parts:

FIG. 1 illustrates an exemplary wireless network that transmits messagesin the uplink according to the principles of the present disclosure;

FIG. 2 is a high-level diagram of an OFDMA transmitter according to oneembodiment of the disclosure;

FIG. 3 is a high-level diagram of an OFDMA receiver according to oneembodiment of the disclosure;

FIGS. 4A and 4B illustrate dedicated reference signal patterns accordingto an embodiment of this disclosure;

FIG. 5 illustrates a rotated dedicated reference signal patternaccording to an embodiment of this disclosure;

FIG. 6 illustrates power imbalance across OFDM symbols;

FIG. 7 illustrates a Walsh cover mapping according to an embodiment ofthis disclosure;

FIGS. 8A through 8C illustrate a Walsh cover mapping according toanother embodiment of this disclosure;

FIG. 9 illustrates a method of providing power balancing across OFDMsymbols according to an embodiment of this disclosure;

FIGS. 10A through 10B illustrate a balance of powers in four OFDMsymbols according to an embodiment of this disclosure;

FIG. 11 is a table of scaling factors used for reference signal scalingfor rank-2 transmission according to an embodiment of this disclosure;

FIG. 12 is a table of scaling factors used for reference signal scalingfor rank-3 transmission according to an embodiment of this disclosure;

FIG. 13 is a table of scaling factors used for reference signal scalingfor rank-2, 3 and 4 transmissions according to an embodiment of thisdisclosure;

FIG. 14 is a table of scaling factors used for reference signal scalingfor rank-2, 3 and 4 transmissions according to another embodiment ofthis disclosure;

FIG. 15 is a table illustrating a construction of a resource blockaccording to an embodiment of this disclosure;

FIGS. 16A through 16B illustrate a balance of powers in four OFDMsymbols according to another embodiment of this disclosure;

FIG. 17 is a table of scaling factors used for reference signal scalingfor rank-2 transmission according to another embodiment of thisdisclosure;

FIG. 18 illustrates a method of operating a base station according to anembodiment of this disclosure; and

FIG. 19 illustrates a method of operating a subscriber station accordingto an embodiment of this disclosure.

DETAILED DESCRIPTION

FIGS. 1 through 19, discussed below, and the various embodiments used todescribe the principles of the present disclosure in this patentdocument are by way of illustration only and should not be construed inany way to limit the scope of the disclosure. Those skilled in the artwill understand that the principles of the present disclosure may beimplemented in any suitably arranged wireless communication system.

With regard to the following description, it is noted that the LTE term“node B” is another term for “base station” used below. Also, the LTEterm “user equipment” or “UE” is another term for “subscriber station”used below.

FIG. 1 illustrates exemplary wireless network 100, which transmitsmessages according to the principles of the present disclosure. In theillustrated embodiment, wireless network 100 includes base station (BS)101, base station (BS) 102, base station (BS) 103, and other similarbase stations (not shown).

Base station 101 is in communication with Internet 130 or a similarIP-based network (not shown).

Base station 102 provides wireless broadband access to Internet 130 to afirst plurality of subscriber stations within coverage area 120 of basestation 102. The first plurality of subscriber stations includessubscriber station 111, which may be located in a small business (SB),subscriber station 112, which may be located in an enterprise (E),subscriber station 113, which may be located in a WiFi hotspot (HS),subscriber station 114, which may be located in a first residence (R),subscriber station 115, which may be located in a second residence (R),and subscriber station 116, which may be a mobile device (M), such as acell phone, a wireless laptop, a wireless PDA, or the like.

Base station 103 provides wireless broadband access to Internet 130 to asecond plurality of subscriber stations within coverage area 125 of basestation 103. The second plurality of subscriber stations includessubscriber station 115 and subscriber station 116. In an exemplaryembodiment, base stations 101-103 may communicate with each other andwith subscriber stations 111-116 using OFDM or OFDMA techniques.

While only six subscriber stations are depicted in FIG. 1, it isunderstood that wireless network 100 may provide wireless broadbandaccess to additional subscriber stations. It is noted that subscriberstation 115 and subscriber station 116 are located on the edges of bothcoverage area 120 and coverage area 125. Subscriber station 115 andsubscriber station 116 each communicate with both base station 102 andbase station 103 and may be said to be operating in handoff mode, asknown to those of skill in the art.

Subscriber stations 111-116 may access voice, data, video, videoconferencing, and/or other broadband services via Internet 130. In anexemplary embodiment, one or more of subscriber stations 111-116 may beassociated with an access point (AP) of a WiFi WLAN. Subscriber station116 may be any of a number of mobile devices, including awireless-enabled laptop computer, personal data assistant, notebook,handheld device, or other wireless-enabled device. Subscriber stations114 and 115 may be, for example, a wireless-enabled personal computer(PC), a laptop computer, a gateway, or another device.

FIG. 2 is a high-level diagram of an orthogonal frequency divisionmultiple access (OFDMA) transmit path 200. FIG. 3 is a high-leveldiagram of an orthogonal frequency division multiple access (OFDMA)receive path 300. In FIGS. 2 and 3, the OFDMA transmit path 200 isimplemented in base station (BS) 102 and the OFDMA receive path 300 isimplemented in subscriber station (SS) 116 for the purposes ofillustration and explanation only. However, it will be understood bythose skilled in the art that the OFDMA receive path 300 may also beimplemented in BS 102 and the OFDMA transmit path 200 may be implementedin SS 116.

The transmit path 200 in BS 102 comprises a channel coding andmodulation block 205, a serial-to-parallel (S-to-P) block 210, a Size NInverse Fast Fourier Transform (IFFT) block 215, a parallel-to-serial(P-to-S) block 220, an add cyclic prefix block 225, an up-converter (UC)230, a reference signal multiplexer 290, and a reference signalallocator 295.

The receive path 300 in SS 116 comprises a down-converter (DC) 255, aremove cyclic prefix block 260, a serial-to-parallel (S-to-P) block 265,a Size N Fast Fourier Transform (FFT) block 270, a parallel-to-serial(P-to-S) block 275, and a channel decoding and demodulation block 280.

At least some of the components in FIGS. 2 and 3 may be implemented insoftware while other components may be implemented by configurablehardware or a mixture of software and configurable hardware. Inparticular, it is noted that the FFT blocks and the IFFT blocksdescribed in the present disclosure document may be implemented asconfigurable software algorithms, where the value of Size N may bemodified according to the implementation.

Furthermore, although the present disclosure is directed to anembodiment that implements the Fast Fourier Transform and the InverseFast Fourier Transform, this is by way of illustration only and shouldnot be construed to limit the scope of the disclosure. It will beappreciated that in an alternate embodiment of the disclosure, the FastFourier Transform functions and the Inverse Fast Fourier Transformfunctions may easily be replaced by Discrete Fourier Transform (DFT)functions and Inverse Discrete Fourier Transform (IDFT) functions,respectively. It will be appreciated that for DFT and IDFT functions,the value of the N variable may be any integer number (i.e., 1, 2, 3, 4,etc.), while for FFT and IFFT functions, the value of the N variable maybe any integer number that is a power of two (i.e., 1, 2, 4, 8, 16,etc.).

In BS 102, channel coding and modulation block 205 receives a set ofinformation bits, applies coding (e.g., Turbo coding) and modulates(e.g., QPSK, QAM) the input bits to produce a sequence offrequency-domain modulation symbols. Serial-to-parallel block 210converts (i.e., de-multiplexes) the serial modulated symbols to paralleldata to produce N parallel symbol streams where N is the IFFT/FFT sizeused in BS 102 and SS 116. Size N IFFT block 215 then performs an IFFToperation on the N parallel symbol streams to produce time-domain outputsignals. Parallel-to-serial block 220 converts (i.e., multiplexes) theparallel time-domain output symbols from Size N IFFT block 215 toproduce a serial time-domain signal. Add cyclic prefix block 225 theninserts a cyclic prefix to the time-domain signal. Finally, up-converter230 modulates (i.e., up-converts) the output of add cyclic prefix block225 to RF frequency for transmission via a wireless channel. The signalmay also be filtered at baseband before conversion to RF frequency. Insome embodiments, reference signal multiplexer 290 is operable tomultiplex the reference signals using code division multiplexing (CDM)or time/frequency division multiplexing (TFDM). Reference signalallocator 295 is operable to dynamically allocate reference signals inan OFDM signal in accordance with the methods and system disclosed inthe present disclosure.

The transmitted RF signal arrives at SS 116 after passing through thewireless channel and reverse operations performed at BS 102.Down-converter 255 down-converts the received signal to basebandfrequency and remove cyclic prefix block 260 removes the cyclic prefixto produce the serial time-domain baseband signal. Serial-to-parallelblock 265 converts the time-domain baseband signal to parallel timedomain signals. Size N FFT block 270 then performs an FFT algorithm toproduce N parallel frequency-domain signals. Parallel-to-serial block275 converts the parallel frequency-domain signals to a sequence ofmodulated data symbols. Channel decoding and demodulation block 280demodulates and then decodes the modulated symbols to recover theoriginal input data stream.

Each of base stations 101-103 may implement a transmit path that isanalogous to transmitting in the downlink to subscriber stations 111-116and may implement a receive path that is analogous to receiving in theuplink from subscriber stations 111-116. Similarly, each one ofsubscriber stations 111-116 may implement a transmit path correspondingto the architecture for transmitting in the uplink to base stations101-103 and may implement a receive path corresponding to thearchitecture for receiving in the downlink from base stations 101-103.

The total bandwidth in an OFDM system is divided into narrowbandfrequency units called subcarriers. The number of subcarriers is equalto the FFT/IFFT size N used in the system. In general, the number ofsubcarriers used for data is less than N because some subcarriers at theedge of the frequency spectrum are reserved as guard subcarriers. Ingeneral, no information is transmitted on guard subcarriers.

The transmitted signal in each downlink (DL) slot of a resource block isdescribed by a resource grid of N_(RB) ^(DL)N_(sc) ^(RB) subcarriers andN_(symb) ^(DL) OFDM symbols. The quantity N_(RB) ^(DL) depends on thedownlink transmission bandwidth configured in the cell and fulfillsN_(RB) ^(min,DL)≦N_(RB) ^(DL)≦N_(RB) ^(max,DL), where N_(RB) ^(min,DL)and N_(RB) ^(max,DL) are the smallest and largest downlink bandwidth,respectively, supported. In some embodiments, subcarriers are consideredthe smallest elements that are capable of being modulated.

In case of multi-antenna transmission, there is one resource griddefined per antenna port.

Each element in the resource grid for antenna port p is called aresource element (RE) and is uniquely identified by the index pair (k,l)in a slot where k=0, . . . , N_(RB) ^(DL)N_(sc) ^(RB)−1 and l=0, . . . ,N_(symb) ^(DL)−1 are the indices in the frequency and time domains,respectively. Resource element (k,l) on antenna port p corresponds tothe complex value a_(k,l) ^((p)). If there is no risk for confusion orno particular antenna port is specified, the index p may be dropped.

In LTE, DL reference signals (RSs) are used for two purposes. First, UEsmeasure channel quality information (CQI), rank information (RI) andprecoder matrix information (PMI) using DL RSs. Second, each UEdemodulates the DL transmission signal intended for itself using the DLRSs. In addition, DL RSs are divided into three categories:cell-specific RSs, multi-media broadcast over a single frequency network(MBSFN) RSs, and UE-specific RSs or dedicated RSs (DRSs).

Cell-specific reference signals (or common reference signals: CRSs) aretransmitted in all downlink subframes in a cell supporting non-MBSFNtransmission. If a subframe is used for transmission with MBSFN, onlythe first a few (0, 1 or 2) OFDM symbols in a subframe can be used fortransmission of cell-specific reference symbols. The notation R_(p) isused to denote a resource element used for reference signal transmissionon antenna port p.

UE-specific reference signals (or dedicated RS: DRS) are supported forsingle-antenna-port transmission of Physical Downlink Shared Channel(PDSCH) and are transmitted on antenna port 5. The UE is informed byhigher layers whether the UE-specific reference signal is present and isa valid phase reference for PDSCH demodulation or not. UE-specificreference signals are transmitted only on the resource blocks upon whichthe corresponding PDSCH is mapped.

The time resources of an LTE system are partitioned into 10 millisecond(msec) frames, and each frame is further partitioned into 10 subframesof one msec duration each. A subframe is divided into two time slots,each of which spans 0.5 msec. A subframe is partitioned in the frequencydomain into multiple resource blocks (RBs), where an RB is composed of12 subcarriers.

For dual-layer beamforming, two sets of dedicated RSs (DRS, a.k.a,DM-RS) are defined for demodulation, where two sets of RSs aremultiplexed in RBs in a subframe by code-division multiplexing (CDM).For example, a CDM RS pattern for dual-layer beamforming is introducedin 3GPP RAN1 contribution R1-090185 “Dual ports DRS design for BF,”CATT, 3GPP TSG RAN WG1 meeting #55bis, January 2009, which is herebyincorporated by reference into the present application as if fully setforth herein.

Furthermore, a CDM/FDM-based pilot pattern that can support up to 4layer transmissions is introduced in R1-090875, “Further Considerationsand Link Simulations on Reference Signals in LTE-A,” Qualcomm Europe,3GPP TSG RAN WG1 meeting #56, February 2009, which is herebyincorporated by reference into the present application as if fully setforth herein.

FIGS. 4A and 4B illustrate dedicated reference signal patterns 410 and420 according to an embodiment of this disclosure.

As shown in FIGS. 4A and 4B, REs labeled with labeled with a letter X,where X is one of G, H, I, J, L, K, are used to carry a number of DRSamong the 8 DRS, where the number of DRS are CDM'ed. Pattern 410 isbased on a spreading factor 2 CDM across two time-adjacent REs with thesame alphabet label, while pattern 420 is based on a spreading factor 4CDM across two groups of two time-adjacent REs with the same alphabetlabel. The 8 antenna ports in a rank-8 transmission pattern are referredto as antenna ports 4, 5, 6, 7, 8, 9, 10, 11 to distinguish them fromthe antenna ports in rank-2 and rank-4 transmission patterns. It isnoted that in Rel-8 LTE, antenna ports 0, 1, 2, 3, 4, 5 are used forCRS, MBSFN RS and Rel-8 DRS. Hence, if the numbering conventionextending Rel-8 LTE is followed, the new antenna port numbers may startfrom 7 Therefore, rank-2 transmission pattern will have antenna ports 7,8. Rank-4 transmission pattern will have antenna ports 7, 8, 9, 10, andrank-8transmission pattern will have antenna ports 11, 12, 13, 14, 15,16, 17, 18.

In one example embodiment of pattern 410, the REs labeled G carry DRS 4,5. The REs labeled H carry DRS 6, 7. The REs labeled I carry DRS 8, 9.The REs labeled J carry DRS 10, 11. On the other hand, in one exampleembodiment of pattern 420, the REs labeled K carry DRS 4, 5, 6, 7 whilethe REs labeled L carry DRS 8, 9, 10, 11.

FIG. 5 illustrates a rotated dedicated reference signal pattern 500according to an embodiment of this disclosure.

Note that when multiple RBs using pattern 410 are allocated, an RBrotation may be used. For example, in one embodiment, even numbered RBsuse pattern 410 as shown in FIGS. 4A and 4B, while odd numbered RBs usea rotated pattern 500 as shown in FIG. 5.

FIG. 6 illustrates power imbalance across OFDM symbols.

When the two DM RS have the same RS sequence, a power-imbalance acrossOFDM symbols occurs after CDM spreading. In FIG. 6, it is assumed thatthe precoding vectors for the two layers associated with the two DM RSare [1 1 1 1] and [1 1 −1 −1], where the entries of the two vectors arefor Tx antenna 1, 2, 3 and 4 from the left to the right. In addition, itis assumed that the RS sequence is [r0 r1 r2 r3 r4 r5 . . . ] and thefirst 6 entries are mapped to the 6 REs of each layer shown in FIG. 6 insubcarriers first, OFDM symbols last order. Here, r0, . . . , r5 arecomplex numbers with magnitude 1. With regard to Tx antenna 1, in bothlayers, three REs 601, 603, 605 on a first OFDM symbol with DM RS havesignals r0, r1, r2, respectively, from the top to the bottom in bothlayer 0 and layer 1. However, the REs 607, 609, 611 on a second,adjacent OFDM symbol have signals r3, r4, r5 in layer 0, while the REs613, 615, 617 have signals −r3, −r4 and −r5 in layer 1. Hence, the sumpower across the three subcarriers on the first OFDM symbol is 4, andthe sum power on the second OFDM symbol is 0. However, it is desirableto keep a consistent power radiation in OFDM symbols.

Accordingly, this disclosure provides a method and system for balancingout RS powers across OFDM symbols with DM RS, for various DM RSpatterns.

In one embodiment of this disclosure, a DM RS corresponding to a firsttransmission layer (or a transmission stream) is considered. In onesubcarrier with the DM RS, a Walsh cover W=[W₀, W₁, . . . W_(N)] isassigned. Then, in a closest subcarrier with the DM RS, a variation ofthe Walsh cover W, e.g. W′, is assigned. Examples of the variation ofthe Walsh cover are:

W′ is constructed by flipping the sign of W. In other words, W′=−W;

W′ is constructed by cyclically shift the elements in W to the right. Inother words, W′=[W_(N), W₀, W₁, . . . , W_(N-1)]; and

W′ is constructed by cyclically shift the elements in W to the left. Inother words, W′=[W₁, . . . , W_(N), W₀].

In some embodiments, a second Walsh cover U is used across all thesubcarriers for a second DM RS corresponding to a second transmissionlayer.

FIG. 7 illustrates a Walsh cover mapping 700 according to an embodimentof this disclosure.

In an embodiment of this disclosure, when a rank-2 DM RS pattern is usedwithin an RB, Walsh cover [1 −1] is used for DM RS 1 and a variation ofthe Walsh cover [1 −1] is applied. Also, Walsh cover [1 1] is used forDM RS 0, but a variation of the Walsh cover [1 1] is not applied. Inthis particular embodiment, the variation of Walsh cover [1 −1] is [−11]. The resultant Walsh cover mapping in DM RS REs in an RB is shown asthe Walsh cover mapping 700, which illustrates Walsh cover variation inthe frequency domain.

FIGS. 8A through 8C illustrate a Walsh cover mapping 800 according toanother embodiment of this disclosure.

In another embodiment of this disclosure, at least two RBs are assignedto a UE, where at least two of the at least two RBs are adjacent orconsecutive in the frequency domain. For the at least two adjacent RBs,an eNodeB assigns two layers together with two DM RS in a rank-2 DM RSpattern, where each layer is precoded with a precoding vector. Walshcover [1 −1] is used for DM RS 1, and a variation of Walsh cover [1 −1]is applied. Walsh cover [1 1] is used for DM RS 0, but a variation ofWalsh cover [1 1] is not applied. In this particular embodiment, thevariation of Walsh cover [1 −1] is [−1 1]. The resultant Walsh covermapping in DM RS REs in the adjacent or consecutive RBs is shown as theWalsh cover mapping 800, which illustrates Walsh cover variation in thefrequency domain.

In the Walsh cover mapping 800, for layer 1, a Walsh cover and a variedWalsh cover [1 −1] and [−1 1] are alternating across subcarriers with acorresponding DM RS for layer 1. FIG. 8C verifies that the power acrosstwo adjacent OFDM symbols with DM RS is balanced for Tx antenna 1.

Although the Walsh covers are described as [1 1] and [1 −1] in the aboveembodiments, one of ordinary skill in the art would recognize that anynumber of Walsh covers may be used.

FIG. 9 illustrates a method 900 of providing power balancing across OFDMsymbols according to an embodiment of this disclosure.

Although FIG. 9 shows only up to two CDM DM RS multiplexing, one ofordinary skill in the art would recognize that the RS scaling disclosedin this embodiment can be applied to any arbitrary number of CDM DM RSmultiplexing. For example, the RS scaling disclosed in this embodimentcan be applied to the 4 CDM DM RS multiplexing of the pattern 420 ofFIG. 4B.

In another embodiment of this disclosure, a plurality of DM RSCDM-multiplexed in a set of DM RS REs in a plurality of RBs is assignedwith N Walsh covers W₁, W₂, . . . , W_(N), where N is the number of DMRS. For example, in the rank-8 transmission pattern 420 of FIG. 4B, upto 4 DM RS are CDM-multiplexed in a set of DM RS REs with 4 Walshcovers.

As shown in FIG. 9, an RS sequence of a length equal to a number of DMRS REs in an RB (the length is 12 in this embodiment) is generated atblock 901. The 12 RS symbols in the RS sequence are mapped to the 12 DMRS REs via a one-to-one mapping at block 903. 3 DM RS symbols assignedin 3 DM RS REs in an m-th OFDM symbol with DM RS are denoted by r_(0m),r_(1m) and r_(2m), where m=0, 1, 2, 3.

At block 905, for each DM RS n, a Walsh spreading is applied on the 4 DMRS symbols, r_(i0), r_(i1), r_(i2) and r_(i3) on a subcarrier with aWalsh cover W_(n)=[W₀, W₁, W₂, W₃], where i=0, 1, 2 and n can be 0, 1, 2and 3. If a spreading factor 2 is applied for multiplexing two DM RS,then two Walsh covers are applied. For example, W₀=[1,1,1,1] is used forDM RS 0, and W₁=[1,−1,1,−1] is used for DM RS 1. If a spreading factor 4is applied for multiplexing four DM RS such as in the rank-8transmission pattern 420 in FIG. 4B, then four Walsh covers are applied.For example, W₀=[1,1,1,1] is used for DM RS 0, W₁=[1,−1,1,−1] is usedfor DM RS 1, W₀=[1,1,−1,−1] is used for DM RS 2, and W₁=[1,−1,−1,1] isused for DM RS 3. For example, for DM RS 1 (or for layer 1), the RSsymbols mapped to the 12 RS REs after spreading with W₁=[1,−1,1,−1] are[r_(i0),−r_(i1),r_(i2),−r_(i3)] in each subcarrier i.

After Walsh spreading, reference signal scaling is applied at block 907,so that the power across the OFDM symbols with DM RS is balanced. Foreach DM RS n, RS symbols in each subcarrier i are multiplied spread witha scaling factor f_(in). It is noted that scaling factors are bothlayer-specific and subcarrier-specific.

FIGS. 10A and 10B illustrate a balance of powers in four OFDM symbolsaccording to an embodiment of this disclosure.

Scaling factor f_(in) used at the RS scaling block 907 in FIG. 9 ischosen such that powers in the four OFDM symbols 1001, 1003, 1005, and1007 with RS emitted from each Tx antenna are the same, or at leastsimilar. It is noted that the Walsh spreading block 905 and the scalingblock 907 of FIG. 9 either can be combined into a single block or canswap positions, as long as the input-output relationship in FIG. 9 issatisfied.

Some embodiments of choosing f_(in)'s for CDM-multiplexed DM RS in DM RSpatterns will now be described.

In an embodiment of this disclosure, a resource unit composed of Ksubcarriers, where both data and RS are precoded with one set ofprecoders is considered. Furthermore, there are L subcarriers with RSREs among the K subcarriers, and a multiple of RS are CDM-multiplexed inthe RS REs on each of the L subcarriers. When an eNodeB multiplexes Nstreams in a scheduling unit with N corresponding RS, where N<=L, Ncolumns out of the L columns of an L×L discrete Fourier transform (DFT)matrix can be used for the scaling factors, {f_(in)}, where i=1, L andn=1, . . . N. An example of such an L×L DFT matrix is shown in Equation1 below:

$\begin{matrix}{D_{L \times L} = {\begin{bmatrix}1 & {\mathbb{e}}^{j\frac{2\;{\pi \cdot 0}}{L}} & {\mathbb{e}}^{j\frac{2\;\pi\;{2 \cdot 0}}{L}} & \ldots & {\mathbb{e}}^{j\frac{2\;{{\pi{({L - 1})}} \cdot 0}}{L}} \\1 & {\mathbb{e}}^{j\frac{2\;{\pi \cdot 1}}{L}} & {\mathbb{e}}^{j\frac{2\;\pi\;{2 \cdot 1}}{L}} & \ldots & {\mathbb{e}}^{j\frac{2\;{{\pi{({L - 1})}} \cdot 1}}{L}} \\\vdots & \vdots & \vdots & \ddots & \vdots \\1 & {\mathbb{e}}^{j\frac{2\;{\pi \cdot {({L - 2})}}}{L}} & {\mathbb{e}}^{j\frac{2\;\pi\;{2 \cdot {({L - 2})}}}{L}} & \ldots & {\mathbb{e}}^{j\frac{2\;{{\pi{({L - 1})}} \cdot {({L - 2})}}}{L}} \\1 & {\mathbb{e}}^{j\frac{2\;{\pi \cdot {({N - 1})}}}{L}} & {\mathbb{e}}^{j\frac{2\;\pi\;{2 \cdot {({L - 1})}}}{L}} & \ldots & {\mathbb{e}}^{j\frac{2\;{{\pi{({L - 1})}} \cdot {({L - 1})}}}{L}}\end{bmatrix}.}} & \lbrack {{Eqn}.\mspace{14mu} 1} \rbrack\end{matrix}$

In a particular embodiment, the elements of {f_(in)} are listed in amatrix as shown in Equation 2 below:

$\begin{matrix}{\{ f_{in} \} = {\begin{bmatrix}f_{11} & \ldots & f_{1\; N} \\\vdots & \ddots & \vdots \\f_{L\; 1} & \ldots & f_{LN}\end{bmatrix}.}} & \lbrack {{Eqn}.\mspace{14mu} 2} \rbrack\end{matrix}$

In some embodiments, this choice of {f_(in)} balances out the poweracross OFDM symbols with CDM RS REs.

In a particular embodiment, a resource unit is an RB composed of K=12subcarriers. In addition, L=3 subcarriers have RS REs such as in FIGS. 4and 10. In such an embodiment, a 3×3 DFT matrix can be defined as shownin Equation 3 below:

$\begin{matrix}{{D_{3 \times 3} = {\begin{bmatrix}1 & 1 & 1 \\1 & \omega & \omega^{2} \\1 & \omega^{2} & \omega^{4}\end{bmatrix} = \begin{bmatrix}1 & 1 & 1 \\1 & \omega & \omega^{*} \\1 & \omega^{*} & \omega\end{bmatrix}}},} & \lbrack {{Eqn}.\mspace{14mu} 3} \rbrack\end{matrix}$where ω is defined as

$\omega = {{\mathbb{e}}^{j\frac{2\;\pi}{3}} = \frac{{- 1} + \sqrt{3\; i}}{2}}$and ω* is a complex conjugate of ω. In such an embodiment, it can beverified that ω*+ω+1=0.

FIG. 11 is a table 1100 of scaling factors used for reference signalscaling for rank-2 transmission according to an embodiment of thisdisclosure.

In an embodiment of this disclosure, two layers (and two DM RS) aremultiplexed within an RB. The two DM RS are multiplexed in RS REs on asubcarrier with two Walsh covers, W₀=[1,1,1,1] for DM RS 0,W₁=[1,−1,1,−1] for DM RS 1. It is noted that this choice of CDM Walshcovers can also be interpreted as a spreading factor 2, as [1 1] and [1−1] are repeated twice in W₀ and W₁. In such an embodiment, one set off_(in)'s used at the RS scaling block 907 in FIG. 9 that balances thesum powers across all four OFDM symbols, where the sum is taken placeover the three DM RS REs in each OFDM symbol in an RB, for example,shown in FIGS. 10A-10B, is shown in the table 1100. It is noted that forlayers 0 and 1 in the table 1100, other combinations and permutations oftwo columns may be chosen out of the columns, for example, from the DFTmatrix D_(3×3) of Equation 3. It is also noted that the two Walsh coversW₀=[1,1,1,1] and W [1,−1,1,−1] are just examples and that one ofordinary skill in the art would recognize that any two orthogonal Walshcovers may be chosen for power balancing with two DM RS.

It is noted that the balancing of the sum powers across the four OFDMsymbols in FIGS. 10A-10B can be verified. In a particular embodiment,for a Tx antenna, precoding entries for layers 0 and 1 are A and B,respectively, where A and B are arbitrary complex numbers. Then, the sumpower across the 3 subcarriers at the first and the third OFDM symbols,for example, in FIGS. 10A-10B is shown in Equation 4 below:|A+B| ² +|A+ωB| ² +|A+ω*B| ²=3(|A| ² +|B| ²)+2Re(A*B(1+ω+ω*))=3(|A| ²+|B| ²).  [Eqn. 4]

Furthermore, the sum power across the 3 subcarriers at the second andthe fourth OFDM symbols is shown in Equation 5 below:|A−B| ² +|A−ωB| ² +|A−ω*B| ²=3(|A| ² +|B| ²)−2Re(A*B(1+ω+ω*))=3(|A| ²+|B| ²).  [Eqn. 5]

In this particular embodiment, the two numbers are identical to3(|A|²+|B|²), which is a triple of a power in a RE with two precoded DMRS symbols. Accordingly, the powers are balanced across the four OFDMsymbols within an RB in this embodiment.

FIG. 12 is a table 1200 of scaling factors used for reference signalscaling for rank-3 transmission according to an embodiment of thisdisclosure.

In another embodiment, three layers (and three DM RS) are multiplexedwithin an RB. The three DM RS are multiplexed in RS REs on a subcarrierwith three Walsh covers, W₀=[1,1,1,1] for DM RS 0, W₁=[1,−1,1,−1] for DMRS 1 and W₂=[1,1,−1,−1] for DM RS 2. Then, one set of f_(in)'s used atthe RS scaling block 907 in FIG. 9 that balances the sum powers acrossall the four OFDM symbols, where the sum is taken place over the threeDM RS REs in each OFDM symbol in an RB, for example, shown in FIGS.10A-10B, is shown in the table 1200. It is noted that any permutation ofthe columns can also be used for f_(in)'s. It is also noted that thethree Walsh covers W₀=[1,1,1,1], W₁=[1,−1,1,−1] and W₂=[1,1,−1,−1] arejust examples and that one of ordinary skill in the art would recognizethat any three orthogonal Walsh covers may be chosen for power balancingwith three DM RS.

It is noted that the balancing of the sum powers across the four OFDMsymbols in FIGS. 10A-10B can be verified. In another embodiment, fourlayers (and four DM RS) are multiplexed within an RB. The four DM RS aremultiplexed in RS REs on a subcarrier with four Walsh covers. Then, oneset of f_(in)'s that mitigates the power imbalance across the four OFDMsymbols, where the sum is taken place over the three DM RS REs in eachOFDM symbol in an RB, for example, shown in FIGS. 10A-10B, isconstructed by adding one more column for Layer 3 to {f_(in)} with threecolumns, e.g., the table 1200. One set of examples of an additionalcolumn for layer 3 is constructed with a unit-amplitude complex numberentries:

$\begin{bmatrix}1 \\{- 1} \\1\end{bmatrix},\begin{bmatrix}{- 1} \\1 \\{- 1}\end{bmatrix},{{{and}\mspace{14mu}\begin{bmatrix}1 \\{- 1} \\{- 1}\end{bmatrix}}.}$Another set of examples of an additional column for layer 3 isconstructed with one column from the DFT matrix, for example, shown inEquation 3:

$\begin{bmatrix}1 \\1 \\1\end{bmatrix},\begin{bmatrix}1 \\\omega \\\omega^{*}\end{bmatrix},{{{and}\mspace{14mu}\begin{bmatrix}1 \\\omega^{*} \\\omega\end{bmatrix}}.}$

In other embodiments of this disclosure, in a resource unit, there areat least L=6 subcarriers having RS REs. In this case, a 6×6 DFT matrixshown in Equation 6 below is considered:

$\begin{matrix}{D_{6 \times 6} = {\begin{bmatrix}1 & 1 & 1 & 1 & 1 & 1 \\1 & \omega^{1/2} & \omega & \omega^{3/2} & \omega^{2} & \omega^{5/2} \\1 & \omega & \omega^{2} & \omega^{3} & \omega^{4} & \omega^{5} \\1 & \omega^{3/2} & \omega^{3} & \omega^{9/2} & \omega^{6} & \omega^{15/2} \\1 & \omega^{2} & \omega^{4} & \omega^{6} & \omega^{8} & \omega^{10} \\1 & \omega^{5/2} & \omega^{5} & \omega^{15/2} & \omega^{10} & \omega^{25/2}\end{bmatrix} = {\quad{\begin{bmatrix}1 & 1 & 1 & 1 & 1 & 1 \\1 & \omega^{1/2} & \omega & {- 1} & \omega^{*} & {- \omega} \\1 & \omega & \omega^{*} & 1 & \omega & \omega^{*} \\1 & {- 1} & 1 & {- 1} & 1 & {- 1} \\1 & \omega^{*} & \omega & 1 & \omega^{*} & \omega \\1 & {- \omega} & \omega^{*} & {- 1} & \omega & \omega^{1/2}\end{bmatrix},}}}} & \lbrack {{Eqn}.\mspace{14mu} 6} \rbrack\end{matrix}$where ω is defined as

$\omega = {{\mathbb{e}}^{j\frac{2\;\pi}{3}} = \frac{{- 1} + \sqrt{3\; i}}{2}}$and ω* is a complex conjugate of ω. Then,

$\omega^{1/2} = {{\mathbb{e}}^{j\frac{\pi}{3}} = {\frac{1 + \sqrt{3\; i}}{2}.}}$In this case, it can be verified that ω*+ω+1=0, ω³=1 and ω^(3/2)=−1. Inone example, a resource unit is at least as large as two consecutive RBscomposed of K=24 subcarriers. In addition, 3 subcarriers have RS REs perRB (or per 12 subcarriers) such as FIGS. 4 and 10.

FIG. 13 is a table 1300 of scaling factors used for reference signalscaling for rank-2, 3 and 4 transmissions according to an embodiment ofthis disclosure.

FIG. 14 is a table 1400 of scaling factors used for reference signalscaling for rank-2, 3 and 4 transmissions according to anotherembodiment of this disclosure.

In one embodiment, up to four layers (and up to four DM RS) aremultiplexed within two consecutive RBs from a bundle of at least twoRBs. The DM RS are multiplexed in RS REs on a subcarrier with Walshcovers. Then, one set of f_(in)'s used at the RS scaling block 907 inFIG. 9 that balances the sum powers across all the four OFDM symbols,where the sum is taken place over the six DM RS REs in each OFDM symbolin the two RBs where an RB, for example, shown in FIGS. 10A-10B, isshown in the table 1300 and the table 1400. It is noted that for thescaling factors for the layers, i.e., {f_(in)}, other combinations andpermutations of four columns may be chosen out of the columns, forexample, from the DFT matrix D_(6×6) of Equation 6. It is also notedthat the scaling factors defined in the table 1400 balance the power perRB up to rank-3 transmission, while the scaling factors require anallocation size of a multiple of two RBs for power balancing for rank-4transmission.

It is noted that the table 1300 and the table 1400 can also be used forscaling 6 subcarriers with DM RS within one RB.

FIG. 15 is a table 1500 illustrating a construction of a resource blockaccording to an embodiment of this disclosure.

In some embodiments of this disclosure, a resource unit composed of 12subcarriers, where both data and RS are precoded with one set ofprecoders, is considered. There are 3 subcarriers with RS REs among the12 subcarriers, and a multiple of RS are multiplexed with CDM in the RSREs on each of the 12 subcarrier. In these embodiments, {f_(in)}, i.e.,

${\{ f_{in} \} = \begin{bmatrix}f_{11} & \ldots & f_{14} \\\vdots & \ddots & \vdots \\f_{31} & \ldots & f_{34}\end{bmatrix}},$is chosen at the RS scaling block 907 in FIG. 9 such that:

the first column is

$\begin{bmatrix}1 \\1 \\1\end{bmatrix},$and

the second, the third and the fourth columns are different from eachother. Each column has two entries with ω and one entry with 1.Alternatively, each column has two entries with ω* and one entry with 1.

It is noted that this solution mitigates the power imbalance problem.

FIGS. 16A and 16B illustrate a balance of powers in four OFDM symbolsaccording to another embodiment of this disclosure.

In the embodiment of FIGS. 10A-10B, each subcarrier with DM RS ismultiplied with two scaling factors at the RS scaling block 907 in FIG.9. The scaling factors are chosen such that powers in the four OFDMsymbols 1601, 1603, 1605, and 1607 with RS emitted from each Tx antennaare the same, or at least similar. One scaling factor is multiplied onthe first two REs in the time domain, and the other is multiplied on thelast two REs in the time domain. For example, as shown in FIGS. 16A-16B,f₀₀ is multiplied to [r₀₀ r₀₁], and f′₀₀ is multiplied to [r₀₂ r₀₃].

FIG. 17 is a table 1700 of scaling factors used for reference signalscaling for rank-2 transmission according to another embodiment of thisdisclosure.

In a particular embodiment, two streams are multiplexed within aphysical resource block (PRB). In this embodiment, f_(in) defined in thetable 1100 and f′_(in) defined in the table 1700 are used to implementthe RS scaling block 907 in FIG. 9. This embodiment increases channelestimation performance at the UE side.

FIG. 18 illustrates a method 1800 of operating a base station accordingto an embodiment of this disclosure.

As shown in FIG. 18, the method 1800 includes transmitting a pluralityof reference signals in a first resource block (block 1801). The method1800 also includes allocating a first layer of the reference signals anda second layer of the reference signals to the same resource elements inthe first resource block (block 1803). The reference signals are (i)allocated to a first group of two adjacent resource elementscorresponding to a first OFDM symbol and a second OFDM symbol on a firstsubcarrier of the first resource block, (ii) allocated to a second groupof two adjacent resource elements corresponding to the first OFDM symboland the second OFDM symbol on a second subcarrier of the first resourceblock, and (iii) allocated to a third group of two adjacent resourceelements corresponding to the first OFDM symbol and the second OFDMsymbol on a third subcarrier of the first resource block. The methodfurther includes multiplexing the first layer of the reference signalswith the second layer of the reference signals by applying a first covercode W1 to the first layer of the reference signals, applying a secondcover code W2, different from the first cover code, to the second layerof the reference signals on the first subcarrier and the thirdsubcarrier, and applying a variation of the second cover code W2′ to thesecond layer of the reference signals on the second subcarrier (block1805).

The method 1800 also includes transmitting the plurality of referencesignals in a second resource block adjacent to the first resource blockin the frequency domain (block 1807). The method 1800 further includesallocating the first layer of the reference signals and the second layerof the reference signals to the same resource elements in the secondresource block (block 1809). The reference signals are (i) allocated toa first group of two adjacent resource elements corresponding to thefirst OFDM symbol and the second OFDM symbol on a first subcarrier ofthe second resource block, (ii) allocated to a second group of twoadjacent resource elements corresponding to the first OFDM symbol andthe second OFDM symbol on a second subcarrier of the second resourceblock, and (iii) allocated to a third group of two adjacent resourceelements corresponding to the first OFDM symbol and the second OFDMsymbol on a third subcarrier of the second resource block. The method1800 still further includes multiplexing the first layer of thereference signals with the second layer of the reference signals byapplying the first cover code W1 to the first layer of the referencesignals, applying the second cover code W2 to the second layer of thereference signals on the second subcarrier, and applying the variationof the second cover code W2′ to the second layer of the referencesignals on the first subcarrier and the third subcarrier (block 1811).

FIG. 19 illustrates a method 1900 of operating a subscriber stationaccording to an embodiment of this disclosure.

As shown in FIG. 19, the method 1900 includes receiving a plurality ofreference signals in a first resource block (block 1901). A first layerof the reference signals and a second layer of the reference signals areallocated to the same resource elements in the first resource block,wherein the reference signals are (i) allocated to a first group of twoadjacent resource elements corresponding to a first OFDM symbol and asecond OFDM symbol on a first subcarrier of the first resource block,(ii) allocated to a second group of two adjacent resource elementscorresponding to the first OFDM symbol and the second OFDM symbol on asecond subcarrier of the first resource block, and (iii) allocated to athird group of two adjacent resource elements corresponding to the firstOFDM symbol and the second OFDM symbol on a third subcarrier of thefirst resource block. The first layer of the reference signals ismultiplexed with the second layer of the reference signals. A firstcover code W1 is applied to the first layer of the reference signals, asecond cover code W2, different from the first cover code, is applied tothe second layer of the reference signals on the first subcarrier andthe third subcarrier, and a variation of the second cover code W2′ isapplied to the second layer of the reference signals on the secondsubcarrier.

The method 1900 also includes receiving a plurality of reference signalsin a second resource block adjacent to the first resource block in thefrequency domain (block 1903). The first layer of the reference signalsand the second layer of the reference signals are allocated to the sameresource elements in the second resource block. The reference signalsare (i) allocated to a first group of two adjacent resource elementscorresponding to the first OFDM symbol and the second OFDM symbol on afirst subcarrier of the second resource block, (ii) allocated to asecond group of two adjacent resource elements corresponding to thefirst OFDM symbol and the second OFDM symbol on a second subcarrier ofthe second resource block, and (iii) allocated to a third group of twoadjacent resource elements corresponding to the first OFDM symbol andthe second OFDM symbol on a third subcarrier of the second resourceblock. The first layer of the reference signals is multiplexed with thesecond layer of the reference signals. The first cover code W1 isapplied to the first layer of the reference signals, the second covercode W2 is applied to the second layer of the reference signals on thesecond subcarrier, and the variation of the second cover code W2′ isapplied to the second layer of the reference signals on the firstsubcarrier and the third subcarrier.

Although the present disclosure has been described with an exemplaryembodiment, various changes and modifications may be suggested to oneskilled in the art. It is intended that the present disclosure encompasssuch changes and modifications as fall within the scope of the appendedclaims.

What is claimed is:
 1. A method of operating a user equipment, themethod comprising: receiving, at the user equipment, reference signalsmapped to resource elements for a first transmission layer or a secondtransmission layer, wherein a first code is applied to first referencesignals for the second transmission layer, the first reference signalsmapped to a first group of the resource elements, and wherein avariation of the first code is applied to second reference signals forthe second transmission layer, the second reference signals mapped to asecond group of the resource elements.
 2. The method of claim 1, whereina second code is applied to reference signals for the first transmissionlayer.
 3. The method of claim 2, wherein the first code is [1 −1], thevariation of first code is [−1 1], and the second code is [1 1].
 4. Themethod of claim 2, wherein the first code is [−1 1], the variation offirst code is [1 −1], and the second code is [1 1].
 5. The method ofclaim 1, wherein the variation of the first code is constructed by atleast one of: flipping a sign of the first code, cyclically shiftingelements of the first code left, or cyclically shifting elements of thefirst code right.
 6. The method of claim 1, wherein the resource elementcomprises two adjacent resource elements.
 7. The method of claim 1,wherein the first group of the resource elements is located on a firstsubcarrier or an eleventh subcarrier in a resource block, and the secondgroup of the resource elements is located on a sixth subcarrier in aresource block.
 8. The method of claim 7, wherein the first group ofresource elements and the second group of resource elements are locatedon fifth and sixth orthogonal frequency division multiplexing (OFDM)symbols in a slot.
 9. A user equipment, comprising: an antennaconfigured to receive reference signals mapped to resource elements fora first transmission layer or a second transmission layer; and one ormore downlink receive path processing circuits configured to determinethe reference signals, wherein a first code is applied to a first of thereference signals for the second transmission layer, the first referencesignals mapped to a first group of the resource elements, and wherein avariation of the first code is applied to second reference signals forthe second transmission layer, the second reference signals mapped to asecond group of the resource elements.
 10. The user equipment of claim9, wherein a second code is applied to reference signals for the firsttransmission layer.
 11. The user equipment of claim 10, wherein thefirst code is [1 −1], the variation of first code is [−1 1], and thesecond code is [1 1].
 12. The user equipment of claim 10, wherein thefirst code is [−1 1], the variation of first code is [1 −1], and thesecond code is [1 1].
 13. The user equipment of claim 9, wherein thevariation of the first code is constructed by at least one of: flippinga sign of the first code, cyclically shifting elements of the first codeleft, or cyclically shifting elements of the first code right.
 14. Theuser equipment of claim 9, wherein the resource element comprises twoadjacent resource elements.
 15. The user equipment of claim 9, whereinthe first group of the resource elements is located on a firstsubcarrier or an eleventh subcarrier in a resource block, and the secondgroup of the resource elements is located on a sixth subcarrier in aresource block.
 16. The user equipment of claim 15, wherein the firstgroup of resource elements and the second group of resource elements arelocated on fifth and sixth orthogonal frequency division multiplexing(OFDM) symbols in a slot.